Alternate fuels have been developed to mitigate the rising prices of conventional fuels and for reducing exhaust emissions. For example, some gaseous fuels have been recognized as attractive alternative fuels. For automotive applications, natural gas or petroleum gas may be compressed and stored as a liquid (liquefied petroleum gas, or LPG) in cylinders at saturation pressure. LPG predominantly consists of butane and propane, although the exact ratio may vary. As such, the composition of LPG affects the fuel properties of LPG, such as fuel density, saturation pressure, octane rating, etc. Since the fuel properties in turn affect engine operations (e.g., fuel injection amount and timing, etc.), accurate fuel composition estimation is required when using LPG as an automotive fuel.
One example approach for estimating the composition of an LPG fuel is shown by Lee in 2003/0216883. Therein, the LPG fuel composition is calculated from a fuel tank pressure and temperature estimated inside the fuel tank. In particular, the estimated fuel tank pressure and temperature are used to infer saturation vapor pressure data, which in turn is used to estimate the fuel composition.
However, the inventors herein have identified potential issues with such an approach. As one example, fuel tank refill events may affect a fuel composition estimate. The refill induced change in composition may also affect the duration of fuel rail priming required at a subsequent engine restart. As another example, the amount of residual air in the fuel tank may affect the composition estimation. For example, if the amount of the air in the fuel tank is high (e.g., due to the fuel tank not being sufficiently purged during a service event), the partial pressure of the tank air may exceed the saturation pressure of the LPG fuel. The elevated fuel tank pressure may lead to an inaccurate LPG composition estimate. As such, errors in estimating the LPG composition may lead to inaccurate fuel injection as well as insufficient priming, causing degraded engine performance and even engine stalls.
Thus, in one example, some of the above issues may be addressed by a method for an engine operating on a gaseous fuel comprising, inferring a refill event based on a rate of change in fuel tank pressure, and in response to the refill event, selectively updating an estimated fuel composition and priming a fuel rail during a subsequent engine restart. A priming duration may be based on the updated composition. In this way, the accuracy and reliability of a fuel composition update may be improved.
In one example, a refill event for a fuel tank storing a gaseous fuel, such as LPG fuel, may be confirmed based on a rate of change in the fuel tank pressure being higher than a threshold rate. In alternate embodiments, the fuel tank refill event may be inferred based on an increase in the fuel level. Since fuel tank refilling affects the composition of fuel present in the tank, following the refill event, the fuel composition may be updated. By updating the fuel composition while the engine is shutdown, a conservative and most recent fuel composition estimate may be available when the engine is subsequently restarted. The fuel composition may be updated based on fuel tank pressure, temperature and residual air content data. Specifically, the fuel composition is updated only if fuel tank conditions are within defined windows wherein the fuel tank data are reliable. The updated composition may then be used during a subsequent engine restart to determine a priming duration. Specifically, before the engine is cranked and started, the fuel line and rail may be vapor purged by operating the fuel pump for the determined duration to prime the fuel system. As such, this improves engine startability. In addition, fuel injection timing and amounts at the engine start may be adjusted based on the updated composition. If the fuel composition cannot be updated due to fuel tank conditions not permitting the update, a most recent, un-updated, composition estimate may be retained and used to adjust the priming duration along with a correction factor.
It will be appreciated that the gaseous fuel referred to herein is a fuel that is gaseous at atmosphere conditions but may be in liquid form while at high pressure (specifically, above saturation pressure) in the fuel system.
In this way, a composition update may be enabled only upon ascertaining that the tank data is not erroneous, or subject to corruption. Consequently, a fuel composition may be estimated post an extended engine shutdown condition with higher accuracy and reliability. By using the more accurate fuel composition estimate to adjust priming operations before an engine start, and injector operations following the engine start, engine performance may be improved. In addition, engine stalls due to insufficient fuel rail priming may be reduced.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.